The invention relates to a method for operating a spark-ignition internal combustion engine, in particular a spark-ignition internal combustion engine with direct fuel injection in the post-start phase at low temperatures.
In a conventional injection strategy in direct-injection gasoline engines and gasoline engines with intake pipe injection, the engine is operated in the so-called homogeneous operating mode in the initial seconds after a cold start at low temperatures of −30° C. to 0° C. Here, in the direct-injection gasoline engine, the injection into the combustion chamber takes place during the intake stroke. As a result of the intake air flowing in and the flows generated in the combustion chamber by the downward movement of the piston, the injected fuel is mixed uniformly with the fresh air. The charge thus generated in the combustion chamber is completely homogenized, that is to say substantially the same air/fuel mixture is present in the entire combustion chamber.
In order to ensure reliable combustion of the fresh mixture, the air/fuel mixture in the vicinity of the spark plug must be slightly rich, that is to say have a slight excess of fuel, at the time of ignition. However, since the mixture is uniformly/homogeneously distributed throughout the entire combustion chamber in the homogeneous operating mode, a slightly rich mixture must be generated in the entire combustion chamber. As a result of the lack of oxygen during combustion, this brings about increased emissions of carbon monoxide and unburned hydrocarbons. The incomplete transformation of the energy in the fuel also results in increased fuel consumption.
During a cold start at very low temperatures, fuel condenses out of the fresh charge onto the still-cold combustion chamber walls and onto the piston head in the post-start phase. This fuel is then no longer available for combustion and must be compensated for by an increased injection quantity in order to prevent the air/fuel mixture from becoming too lean. An overly lean mixture would, as a result of its worse combustibility at the still-low combustion chamber temperatures after the cold start, lead to rough engine operation or even to instances of misfiring. Neither is acceptable for reasons of driving comfort and pollutant emissions.
The fuel which is condensed onto the combustion chamber wall and onto the piston head is present there in liquid form. Said fuel is no longer involved in the actual torque-generating combustion. The evaporated fuel in the direct vicinity of the cold combustion chamber walls is also no longer involved in the combustion process as a result of so-called “quenching”. Here, the combustion is also deprived of heat and therefore energy by the cold combustion chamber walls. This leads to a reduction in the combustion speed, to the where the flame may be extinguished in the vicinity of the combustion chamber wall. As a result, a layer of unburned hydrocarbons forms in front of the combustion chamber wall. During the exhaust process, the upward-moving piston scrapes the fuel from the combustion chamber wall and pushes it out, unburned, through the open exhaust valve into the exhaust duct. This results in high HC emissions.
The fuel which is not involved in the combustion process must also be compensated for by an increased injection quantity, so that a sufficient fuel and therefore energy quantity for generating the required torque for engine operation is available in those regions of the combustion chamber in which the fresh charge can be involved in the combustion process. Specifically after a cold start at very low temperatures, this requires very high fuel injection quantities which can be a multiple of the injection quantity at full load. In this context, it is also significant that the injection quantity required after the cold start determines the minimum feed quantity of an electric fuel pump or of a high pressure pump.
It is therefore an object of the invention to provide a method for operating an internal combustion engine which avoids the above described problems of a conventional homogeneous operating mode. The method for operating the internal combustion engine in a post-start phase should also in particular reduce the required injection quantity at very low temperatures, permitting a further reduction in pollutant emissions and permitting a required high pressure pump to be relatively small.